The subject of the present invention is a color cathode-ray tube having an internal magnetic screen, and more particularly a cathode-ray tube whose color selection mask is held under tension by a support frame.
A color cathode-ray tube is composed of a glass envelope comprising an approximately rectangular front face joined to a funnel-shaped part terminating in a cylindrical neck. An electron gun is placed in the neck and generates electron beams intended to form a color image on a luminescent screen placed on the internal surface of the front panel. The electron beams are kept in correspondence with the phosphors of the luminescent screen by means of a perforated metal mask called the color selection mask. The mask is attached to an approximately rectangular rigid frame with two pairs of opposed sides—a pair of short sides and a pair of long sides. An internal magnetic screen is generally placed in the funnel-shaped part of the envelope and is joined to the rear of the frame. The primary objective of this magnetic screen is to reduce the influence of the components of the Earth's magnetic field on the trajectories of the electron beams so that the angles of incidence of the said beams on the selection mask are not significantly modified by these components; if this were not so, the points of impact of the beams on the screen would be moved and illuminate phosphors of a color other than that desired.